Brain metastasis in cancer patient is known to relate to poor prognoses. Moreover, it also is known that passage of cancer cells through the blood-brain barrier (BBB) is an important event in brain metastasis, that main constituent cells of the BBB are brain microvascular endothelial cells (BMECs), and that BMECs are connected each other by tight junctions between cells and thereby have very selective permeability. Moreover, the proteins junctional adhesion molecules (JAM-1, JAM-2 and JAM-3), Occludin, Claudins, and Zonula Occludin proteins (ZO-1 and ZO-2) are known to constitute the tight junction. Furthermore, cyclooxygenase-2 (COX-2), the epidermal growth factor receptor (EGFR) ligand HB-EGF, and α 2,6-sialyltransferase (ST6GALNAC5) have been identified as mediators involved in the BBB invasion of cancer cells. However, the early stage molecule mechanisms of cancer cells passing through the BBB have not been revealed (Non Patent Literature 1). A study using a highly metastatic breast cancer cell line MDA-MB-231 has shown that vascular endothelial growth factor (VEGF) is highly expressed in breast cancer cells and increases the permeability of endothelial cells as well as adhesion of cancer cells onto endothelial cells, and thus VEGF is involved in the brain metastasis (Non Patent Literature 2). Although the expression of VEGF has been reported to be necessary for brain metastasis, it has been also reported that the expression of VEGF is not sufficient for brain metastasis, and the destruction mechanism of the BBB in brain metastasis has not been sufficiently explained by VEGF (Non Patent Literature 3).
A mechanism of BBB passage of Cryptococcus neoformans which infects the central nervous system (CNS) has been studied. It has been reported that the adhesion of Cryptococcus to human BMECs causes dephosphorylation of Cofilin, which results in actin rearrangement. This report further indicates that the dephosphorylation of Cofilin by Cryptococcus is controlled via the Rho kinase-LIM kinase-Cofilin pathway (Non Patent Literature 4).
Extracellular vesicles (EVs) including exosomes are known to mediate communication between cells by transporting protein, mRNA, and microRNA (miRNA) contained therein (Non Patent Literature 5). Moreover, it has been reported that extracellular vesicles released from cancer cells are involved in the malignancy of cancer cells in various ways (Non Patent Literature 8), for example, they inhibit the function of natural killer cells (Non Patent Literature 6) and increase the expression of the oncogene protein MET in bone marrow progenitor cells to change the bone marrow progenitor cells toward a pro-metastatic phenotype (Non Patent Literature 7). As seen above, the relation between EVs and cancer metastasis has been suggested. However, little was known about the relation between EVs (and substances contained therein) and the BBB.